1. Technical Field
This invention relates in general to computer graphics display systems, and more particularly, to a processing method for identifying a displayed object that intersects an operator selected area of the display screen and for enlarging a selected area of the display screen to facilitate operator selection of an object when a complex display is presented.
2. Description of the Prior Art
Interactive raster graphics systems, such as CAD/CAM (Computer Aided Design/Computer Aided Manufacture) workstations, are widely used to design components and systems of mechanical, electrical, electromechanical and electronics devices. Frequently, the emphasis within such systems is on an operators interacting with a computer based model of a component or system being designed in order to test, for example, its mechanical, electrical or thermal properties. The computer based model is comprised of numerous graphics objects that are individually processed and displayed for operator action. Selection of a displayed object is accomplished via any one of a number of operator controlled interaction devices, such as light pens, locators (e.g., a data tablet with stylus), and alphanumeric and function keyboards. An important part of many interaction sequences is computer identification of an operator selected displayed object to be operated upon, a process which is complicated by the pipeline processing techniques utilized within most graphics systems today.
Typically, graphics system processing techniques require the reprocessing of an entire display screen to identify a particular displayed object selected or picked for further processing. More particularly, existing processing methods require the reexecution of the whole display program, including: transformation of each geometric primitive defining a displayed object in world coordinate space; clipping of each transformed primitive against the predefined clipping boundary in world coordinate space; mapping of each clipped primitive to an operator defined viewport in screen coordinate space; rasterization of all mapped data; and finally, determination of whether the generated pixels intersect the operator defined selection area or window in screen coordinate space. If so, then a pick occurs.
Although effective, a clear drawback to this procedure is the unnecessary processing occuring as the result of mapping and rasterizing geometric objects not selected. For example, rasterization of a filled polygon outside the operator selected screen area is both unnecessary and time consuming. A similar processing technique is also disadvantageously used in most graphics systems to enlarge or "zoom out" an operator selected area of a complex display screen to facilitate selection of a particular displayed object Again, rerasterization of objects in nonselected areas of the display screen ultimately comprises unnecessary processing. Thus, a genuine need clearly exists for a more efficient interactive raster graphics display system processing technique for identifying an operator selected displayed object to be operated on and for zooming out an operator choosen area of the display screen to facilitate picking of a displayed object.